This invention relates to a method for preparing hexafluoropropene oxide (referred to as HFPO, hereinafter) polymers, and more particularly to a method for preparing essentially difunctional HFPO polymers having a minimized content of monofunctional HFPO polymer.
One prior art method for preparing difunctional HFPO polymers is described in U.S. Pat. No. 3,250,807. This method is to react FOCxe2x80x94(CF2)nxe2x80x94COF wherein n is from 0 to 6 with HFPO in an aprotic polar solvent in the presence of a catalyst such as an alkali metal fluoride or activated carbon, thereby forming difunctional HFPO polymers, as shown by the following reaction scheme. 
An attempt to add HFPO to previously furnished xe2x80x94COF groups as above, however, often gives rise to the problem that chain transfer side reaction occurs to form a HFPO polymer having a hexafluoropropyl group at one end (monofunctional HFPO polymer) as shown by the following scheme. 
An improved method for preparing essentially difunctional HFPO polymers while preventing such chain transfer is disclosed in U.S. Pat. No. 3,660,315 or JP-B 53-5360. This method involves mixing the compound of the formula:
FOCCF(CF3 )OCF2CF2OCF(CF3)COF xe2x80x83xe2x80x83(3) 
with cesium fluoride in tetraethylene glycol dimethyl ether to form the compound of the formula:
CsOCF2CF(CF3)OCF2CF2OCF(CF3)CF2OCs xe2x80x83xe2x80x83(4), 
and removing the excess of cesium fluoride from the solution, thereby forming a uniform solution, which is used as an initiator for the polymerization of HFPO. Specifically, after the excess of cesium fluoride is separated off, polymerization is effected at a low temperature of xe2x88x9260xc2x0 C. to xe2x88x9230xc2x0 C., thereby forming pure difunctional HFPO polymers having a number average molecular weight of about 50.
However, it is described in J. Macromol. Sci. Chem., A8(3), 499 (1974) that if the molar ratio of HFPO to the initiator is increased in order to produce difunctional HFPO polymers having a higher degree of polymerization, the side reaction to produce a monofunctional HFPO polymer increases and the purity of difunctional HFPO polymers lowers.
U.S. Pat. No. 4,356,291 or JP-A 57-175185 describes that a HFPO polymer having a number average molecular weight of 445 is obtained using highly purified HFPO in addition to the above initiator. It is pointed out that HFPO generally contains impurities such as hydrogen fluoride, acid fluorides and water, which limit the maximum degree of polymerization of polymers resulting from polymerization of HFPO. Then by subjecting highly purified HFPO to polymerization, a HFPO polymer having a high molecular weight is produced. However, no reference is made therein to the by-produced monofunctional HFPO polymer and the purity of the desired difunctional HFPO polymer.
Understandably, the prior studies on difunctional HFPO polymers placed a main focus on the reduction of undesired monofunctional HFPO polymers resulting from chain transfer and the formation of HFPO polymers having a high degree of polymerization.
All these methods, however, have the drawback that the compound of formula (3) itself contains monofunctional impurities. More particularly, the compound of formula (3) is generally prepared by the following method. 
Upon reaction of oxalic fluoride with HFPO, there are produced not only the end compound of formula (3), but also HFPO oligomers as shown by formulas (3xe2x80x2) and (3xe2x80x3). A precise distillation operation is necessary to separate these oligomers from the end compound. Still worse, the end compound purified by such a precise distillation operation yet contains about 4 to 6% by weight of the monofunctional component having a cyclic structure shown by the above formula (5). Undesirably, since this by-product of formula (5) has the same molecular weight as the end compound of formula (3), it is almost impossible in practice to separate the by-product by further distillation. Use of the fraction resulting from distillation as the initiator means that the monofunctional component already exists prior to the polymerization of HFPO.
On the other hand, known perfluorodicarboxylic fluorides include perfluoroadipic fluoride, perfluoroglutaric fluoride and perfluorosuccinic fluoride. If a polymerization initiator is prepared from these compounds in the same manner as the compound of the above formula (3), side reaction such as esterification can take place, failing to obtain an alcoholate equivalent to the perfluorodicarboxylic fluoride added. If polymerization of HFPO is carried out using this polymerization initiator, there are produced polymers having a wide molecular weight distribution because of the increased content of low molecular weight components. This polymerization initiator is inadequate.
Under the circumstances, it is desired in the polymerization of HFPO to prepare a polymerization initiator using a starting reactant which is available at a relatively low cost, which quantitatively forms an alcoholate with an alkali metal fluoride in an aprotic polar solvent and which is free of monofunctional impurities.
An object of the invention is to provide a method for preparing difunctional HFPO polymers having a minimized content of monofunctional HFPO polymer, using a polymerization initiator prepared from a starting reactant which is available at a relatively low cost.
We have found that when a perfluorodicarboxylic fluoride or perfluorodiketone of the general formula (1) or (2) shown below is mixed with an alkali metal fluoride in an aprotic polar solvent, there is obtained a uniform solution in which a quantitative amount of an alcoholate is formed. Subsequent polymerization of HFPO using this solution as a polymerization initiator results in a difunctional HFPO polymer which is substantially free from the terminal ether structure based on the compound of the above formula (5), has a narrow molecular weight distribution, and has a minimized content of monofunctional HFPO polymer.
Accordingly, the invention provides a method for preparing a hexafluoropropene oxide polymer comprising the steps of mixing a perfluorodicarboxylic fluoride or perfluorodiketone of the following general formula (1) or (2) with a metal fluoride in an aprotic polar solvent, and feeding hexafluoropropene oxide to the resulting solution. 
Herein Rf is a perfluoroalkylene group which may be separated by an oxygen atom, and R1, which may be the same or different, is a perfluoroalkyl group of 1 to 8 carbon atoms.
The method for preparation of HFPO polymers according to the invention uses as the polymerization initiator a solution which is prepared by mixing a perfluorodicarboxylic fluoride or perfluorodiketone of the general formula (1) or (2) with an alkali metal fluoride in an aprotic polar solvent. More particularly, the polymerization initiator is prepared by suspending an alkali metal fluoride in an aprotic polar solvent, adding a perfluorodicarboxylic fluoride or perfluorodiketone to the suspension, and agitating the mixture.
The alkali metal fluoride used herein is preferably cesium fluoride. Examples of the aprotic polar solvent include glymes such as monoglyme, diglyme, triglyme and tetraglyme, tetrahydrofuran and 1,4-dioxane, with the glymes being especially preferred.
With respect to the perfluorodicarboxylic fluoride or perfluorodiketone used herein, an inexpensive hydrocarbon dicarboxylic acid or hydrocarbon diester of the following general formula (6) or (7) is fluorinated by well-known fluorinating methods (including direct fluorination and electrolytic fluorination) to form a corresponding perfluoro compound of the general formula (1) or (2). 
In the formulas, R2 is an alkylene group which may be separated by an oxygen atom, Rf is a perfluoroalkylene group obtained by substituting fluorine atoms for all the hydrogen atoms in R2. R3 is an alkyl group of 1 to 8 carbon atoms, the R3 groups may be the same or different, and R1 is a perfluoroalkyl group of 1 to 8 carbon atoms obtained by substituting fluorine atoms for all the hydrogen atoms in R3. R4 is a substituted or unsubstituted monovalent hydrocarbon group, and the R4 groups may be the same or different.
In formula (1) or (2), Rf is a perfluoroalkylene group which may be separated by an oxygen atom, as exemplified by the following. 
In formula (2), R1 is a perfluoroalkyl group of 1 to 8 carbon atoms, as exemplified by the following:
CcF2c+1xe2x80x94
wherein c is an integer of 1 to 8. These alkyl groups may be straight or branched.
In the practice of the invention, a polymerization initiator solution is prepared by adding the compound of formula (1) or (2) to a mixture of an aprotic polar solvent and an alkali metal fluoride. In the solution, the carbonyl group-bearing compound reacts with the alkali metal fluoride to form a corresponding alcoholate as shown below. The conversion to an alcoholate can be confirmed by infrared absorption analysis of the mixture solution. Depending on the type of the compounds of formulae (1) and (2), the alcoholate may be deposited as solid in the solution at room temperature (for example, 15 to 25xc2x0 C.). In this case, by maintaining the temperature of the solution to 5xc2x0 C. or less, unexpectedly or contrary to the ordinary compounds, the alcoholate may not be deposited in the solution and may be dissolved therein. So, it is preferred to maintain the temperature of the solution to 5xc2x0 C. or less. 
Rf and R1 are as defined above.
The polymerization initiator represented by the above formula is present in the polymerization initiator solution desirably in a concentration of 10 to 60% by weight, especially 25 to 45% by weight.
To the polymerization initiator solution, a second solvent different from the solvent used upon preparation may be added for improving the flow at low temperatures. The second solvent may be one which is uniformly miscible with the initiator solution even at a low temperature below xe2x88x9230xc2x0 C. and has a freezing point of lower than xe2x88x9250xc2x0 C. Desirable are hydrocarbon compounds having one to three ether bonds in the molecule, for example, dimethyl ether, diethyl ether, ethyl methyl ether, methyl propyl ether, ethylene glycol dimethyl ether and tetrahydrofuran. The second solvent is added to reduce the viscosity of the initiator solution at a polymerization temperature in the range of xe2x88x9240xc2x0 C. to xe2x88x9230xc2x0 C. for thereby helping achieve efficient agitation. An appropriate amount of the second solvent added is about 20 to 60 parts by weight per 100 parts by weight of the initiator solution. Preferably the second solvent is previously dried to a water content of 50 ppm or lower.
Next, a perfluoroolefin such as hexafluoropropene (HFP) is reacted with the polymerization initiator solution in optional admixture with the second solvent for forming oligomers. This operation is necessary to remove any chain transfer-inducing substance in the polymerization initiator solution and second solvent, for helping initiate polymerization upon subsequent supply of HFPO.
The perfluoroolefins used herein include those of 2 to 9 carbon atoms, especially 3 to 6 carbon atoms. Examples are given below. 
Of these, the following are especially preferred. 
The amount of the perfluoroolefin used is not critical although it is usually used in an amount of about 0.5 to 100 parts, especially about 3 to 30 parts by weight per 100 parts by weight of the polymerization initiator solution.
Reaction with the perfluoroolefin is usually effected at a temperature of xe2x88x9230xc2x0 C. to 50xc2x0 C., preferably xe2x88x9225xc2x0 C. to 30xc2x0 C. Outside the range, too low a reaction temperature will require a longer time for reaction whereas too high a reaction temperature may cause decomposition of the initiator. The reaction time is not critical although the reaction time at a temperature of xe2x88x9225xc2x0 C. to 30xc2x0 C. is typically about 10 minutes to 2 hours, especially about 20 minutes to 1 hour including the time required for the perfluoroolefin addition.
While the initiator solution, preferably in admixture with the second solvent, is being agitated and cooled in a reactor, HFPO is fed to the reactor, thereby obtaining difunctional HFPO polymers. It is possible to add hexafluoropropene (HFP) at the same time as the HFPO feed. The addition of HFP is effective for increasing flow because it dilutes the reaction solution which gradually thickens with the progress of polymerization. During the polymerization, the reaction solution is preferably kept at a temperature of xe2x88x9245xc2x0 C. to xe2x88x9230xc2x0 C. Below xe2x88x9245xc2x0 C., the reaction solution may increase its viscosity and thixotropy, interfering with efficient agitation. Under such situation, part of the non-flowing reaction product will stick to the reactor inner wall or agitator blade to further interfere with uniform agitation, resulting in polymers having a wider molecular weight distribution. Temperatures above xe2x88x9230xc2x0 C. tend to induce chain transfer reaction to form monofunctional HFPO polymers.
Agitation is important for the reaction solution as a whole to maintain uniform fluidity. A choice is generally made of anchor, paddle, helical ribbon and impeller agitators, depending on the shape and size of the reactor. The number of revolutions is not critical and may be adjusted in accordance with the shape of agitator blade so as to achieve an optimum agitation efficiency.
Preferably the HFPO is continuously fed using a flow meter such as a mass flow controller. A constant rate of HFPO feed is necessary in order to maintain the temperature of the reaction solution in an appropriate range. An appropriate hourly feed rate is about 3 to 15 mol, especially about 5 to 10 mol of HFPO per mol of the initiator. The feed amount is determined as appropriate in accordance with the desired molecular weight and may range from about 30 to 400 mol per mol of the initiator. Since increasing the relative amount of HFPO to a higher level will result in HFPO polymers having a non-negligible amount of monofunctional polymer mixed therein, the feed amount is usually about 30 to 200 mol per mol of the initiator.
The HFP may be fed at the same time as the HFPO and in an amount equal to xc2xc to xc2xe of the weight of HFPO. After the completion of HFPO feed, agitation is continued for a further 1 to 2 hours. Thereafter, the reaction solution is heated and the end product is separated out. In this way, difunctional HFPO polymers of the following formula (8) or (9) are obtained. 
Rf and R1 are as defined above, x and y are positive integers.
The difunctional HFPO polymers of the formula (8) or (9) will contain a minor amount of monofunctional HFPO polymers formed during the reaction process. Since the starting reactant, perfluorodicarboxylic fluoride or perfluorodiketone does contain little of monofunctional impurities, the final content of monofunctional impurities in the difunctional HFPO polymers of the formula (8) or (9) is suppressed dramatically low.
The thus obtained difunctional HFPO polymers are terminated with xe2x80x94COF groups. Then a variety of useful derivatives can be synthesized therefrom by converting the terminal groups into other functional groups. These derivatives will find use in liquid rubber, coating material and sealing material.
Since a perfluorodicarboxylic fluoride or perfluorodiketone which is available at a relatively low cost, which quantitatively forms an alcoholate with an alkali metal fluoride in an aprotic polar solvent and which is free of monofunctional impurities is used as the starting reactant, the invention is successful in producing difunctional HFPO polymers of high purity having a minimized content of monofunctional HFPO polymer and low molecular weight components.